1. Field of the Invention
The present invention relates to stainless steel sheets suitable for use as exterior building materials and methods of manufacturing the same. The present invention is particularly applicable to light-gauge stainless steel sheets having a wall thickness of less than about 0.8 mm and which may be subjected to forming process such as press-forming and roll-forming to manufacture roofing materials having a relatively large surface area.
2. Description of the Prior Art
Hitherto, stainless steel sheets have been used to manufacture exterior building materials, such as sashes, curtain walls and building panels. Generally, stainless steel sheet products for such applications are of a relatively limited surface area.
Recently, stainless steel sheets have found new application as roofing materials, in view of their superior corrosion-resistant weatherproof capability and due to the developments of in-situ forming and roofing technics.
When intended for final use as roofing materials, the stainless sheets are subjected, at any point of time prior to roofing and at any suitable location, to forming process to shape the sheets into desired roofing elements which are mostly in the form of a flanged channel section. To this end, a roll-forming mill, for example, is conveniently installed in the building site and is operated to roll-form the stainless sheet metal into channel-shaped roofing element by bending the sheet metal along the desired bending lines.
Therefore, the material of the stainless steel sheets must exhibit sufficient workability to permit forming. Austenitic stainless steel alloy such as JIS SUS304 stainless steel alloy (18Cr-8Ni) is known as a steel alloy having adequate workability for these purposes and, for this reason, has currently been used to produce stainless steel sheets for roofing materials.
The primary problem with the conventional stainless steel sheets is related to the use of austenitic stainless steel alloy. The production cost is increased because austenitic stainless steel alloy contains a large amount of Ni which is quite expensive. This tends to limit the market of stainless steel sheets as intended for use as exterior building materials, particularly roofing materials.
Another problem with the conventional stainless steel sheets is concerned with the requirement for coating. Currently, stainless steel sheets used for roofing materials are coated with colored coatings. Obviously, this is because it has been believed in the industry that coating of stainless steel sheets is as well necessary in order to avoid the problem experienced with the conventional zinc-plated sheet-iron roof that, once a default occurs in the zinc layer due to deterioration thereof, the underlying sheet iron is subjected to intensive pitting corrosion so that the roof becomes inoperative shortly thereafter due to leakage of rain. In this respect, it has often been pointed out and criticized that investments for expensive stainless steel roof would not be warranted in so far as no one could visually recognize by way of appearance the use of stainless steel sheets as they are concealed by the coating layer applied thereon.
In view of the foregoing, it is desirable that roofing materials made from stainless sheet metal be offered for service in a condition in which the use of stainless steel sheets can readily be visually recognized. In addition, it is desirable to use stainless steel alloy of the class which does not contain expensive Ni. These requirements would be met by making the stainless sheet metal from a ferritic stainless steel alloy and by using the sheet metal as such, i.e., without coating, to provide exterior building materials such as roofing materials.
However, the primary problem which must be overcome in successfully manufacturing the exterior building materials such as roofing materials with the ferritic stainless steel sheets is the formation of "pocket wave" during the forming process. A pocket wave may be defined as a concave depression or convex projection formed on the otherwise flat bottom or side wall of the formed sheet metal product when a sheet metal blank is subjected to forming process, such as roll forming and press forming.
The formation of the pocket wave is related to the workability of the material forming the sheet metal. In the case of the conventional stainless steel sheets made from an austenitic stainless steel alloy, the formation of pocket wave has not been observed to any appreciable degree since the austenitic stainless steel alloy inherently exhibits adequate workability. In contrast, with the currently available stainless steel sheet made from a ferritic stainless steel alloy, there is a tendency of pocket waves being formed to a non-negligible degree. This is intolerable particularly when the stainless steel sheet products are used as roofing materials having a relatively large surface area, because waving of the roof surface due to the presence of the pocket waves on respective roofing elements impairs the attractive appearance of the roof.
Another disadvantage of the currently available sheet metal made from a ferritic stainless steel is that it has poor corrosion resistivity as compared with the austenitic stainless steel. In order to successfully utilize the uncoated ferritic stainless steel sheets as exterior building materials, particularly roofing materials, it is necessarily required that the stainless steel sheets exhibit the outdoor weatherproof capability and corrosion resistivity sufficient to withstand formation of red rust and pitting corrosion for more than 10 years. This is particularly true when the buildings are located in the coastal regions and, therefore, are subjected to saline environment in which airborne saline particles tend to adhere to the roof surface and intensively attack the roofing materials by way of pitting corrosion.